This invention relates generally to measuring apparatus and more particularly to an electronic scale device.
Various electronic scales are commerically available for providing a digital display of the weight of a body placed on a weighing pan. Many of the recently developed scales make use of integrated circuits and microcomputer controls for perfecting various scale operations such as weighing, counting, altering resolution, converting units, etc. In this regard, these devices comprise electronic circuitry usually including a movable transducer and mechanical interfacing means between the electronic circuitry and the weighing pan. The mechanical interfacing mean generally comprises some form of spring means for biasing the pan in a direction opposite to that of the gravitational force exerted upon the pan by the load being weighed.
The movable transducer is typically a variable capacitor comprising a pair of electrically conductive plates, whereby a change in capacitance is effected by moving one of the plates either closer to or further away from the other of the plates responsive to weight being added to or removed from the pan, respectively. In this regard, the variable capacitors used in the prior art generally utilize both a movable plate and a stationary plate such that the movable plate is adjusted to be parallel to the stationary plate when the scale is in a null or starting position (e.g., when no weight is on the scale's weighing pan).
In many of these prior art devices, although the capacitor plates are parallel to each other when the scale is in a null position, the movable plate is arranged to pivot about a rotational axis, thus causing the respective plates to be non-parallel with respect to each other when a load is added to the weighing pan. In this regard, it should be appreciated by those skilled in the relevant art that a scale comprising a variable capacitor constructed of plates which move from a parallel to a non-parallel orientation, although perhaps generally suitable for its intended purpose, requires that rather complex and time consuming mathematical computations be performed in order to relate the amount of weight of the object on the weighing pan to a given change in capacitance. In addition, the process of calibrating the scale tends to be complex and correspondingly time consuming.
Some of the other prior devices not only use capacitors which include plates which pivot from a parallel to a non-parallel orientation but also comprise rotational means which causes at least one of the plates to rotate about a movable as opposed to a fixed axis of rotation. Obviously, this results in the device having to perform mathematical computations of even greater complexity.
In this regard, U.S. Pat. No. 4,273,204 (Gillen) discloses a capacitive weighing scale utilizing a spring which biases a movable capacitor plate against the force of the load being weighed. Although the Gillen device is generally suitable for its intended purpose, its spring comprises a bendable "C"-shaped steel member which orients the respective plates parallel to each other under a first load condition and non-parallel to each other under other load conditions. Furthermore and as should readily be appreciated from the Gillen drawing, the movable plate of the Gillen device rotates about a non-stationary as opposed to a fixed axis of rotation, thus further increasing the complexity of the mathematical operations which must be performed.
Various prior art devices have been constructed in an attempt to simplify the mathematics involved. Certain of these devices utilize a variable capacitor comprising plates which remain parallel to each over their entire operational range of movement. Although this arrangement is generally suitable for its intended purpose, its implementation typically entails a rather complex and correspondingly expensive mechanical arrangement. Other prior art devices attempt to simplify the mathematical complexities by utilizing unusually shaped capacitor plates, e.g., triangular plates. These devices have also tended to be unduly complex in construction and furthermore, the simplified mathematical computations have generally only applied over a limited operational range of movement of the capacitor plate(s).